I focused on the first two seasons, which - in the opinion of myself and many other fans - were the best run. (For reference, these are the episodes that had Lorenzo Music voicing Peter and Laura Summer voicing Janine, and before Slimer began to be more prominent.) I did consider a selection of the best episodes from later seasons as well, but figured I had plenty to work with already.
From those first seasons, I tried to include a mix of the most significant episodes, the best-written episodes, and other episodes I remembered as especially fun or interesting. Because Real Ghostbusters had so many quality episodes, cutting this list down to a single day's worth took some effort, but I managed to get it down to about 8 hours. (I also threw in a few extra episodes* that I figured would be of interest to my audience, ones I wouldn't otherwise have picked, but I won't include all of those below.)
FYI, the ones I consider must-see episodes are in bold.
Season 1 (Saturday morning episodes)
"Mrs. Roger's Neighborhood"
"The Boogieman Cometh"
"Mr. Sandman, Dream Me a Dream"
"When Halloween Was Forever"
Season 2 (syndicated episodes)
"Adventures in Slime and Space"
"Venkman's Ghost Repellers"
"Who're You Calling Two-Dimensional?"
"Chicken, He Clucked"
"You Can't Take It With You"
"No One Comes to Lupusville"
"The Man Who Never Reached Home"
"The Collect Call of Cathulhu"
"The Headless Motorcyclist"
"Janine Melnitz, Ghostbuster"
"The Ghostbusters in Paris"
"Ghost Fight at the O.K. Corral"
"The Long, Long, Long, Etc. Goodbye"
"The Hole in the Wall Gang"
Total uninterrupted runtime: 484 minutes (8 hours, 4 minutes)
* - For reference, the "extra" episodes were Season 2's "Rollerghoster" and "Buster the Ghost", and Season 3's "The Grundel".